Light communication is a wireless communication technology that uses human visible light with frequency among 400 THz to 800 THz. RF bandwidth is a scarce resource. Therefore, visible light communication may provide an alternate technology to meet the strong demands of wireless communications. For example, the visible light emitted from one or more light emitting diodes (LEDs) is widely used in homes and offices, thus it makes the visible light emitted from the one or more LEDs ideal for ubiquitous data transmitters. FIG. 1A and FIG. 1B show schematic views of two exemplary types of white-light LEDs, respectively. FIG. 1A shows an exemplary white-light LED using a blue LED 110 and a yellow phosphor 115. The yellow phosphor 115 limits the modulation bandwidth no more than about 10 Mbps. FIG. 1B shows another exemplary type of a white-light LED using a RGB triplet that may include a blue LED 121, a red LED 122 and a green LED 123, and these three LEDs emit blue light 131, red light 132 and green light 133, respectively. Compared with the white-light LED shown in FIG. 1A, the RGB triplet has a potentially higher bandwidth and the mixing of the triplet may generate any desired colors by any combination of a blue light, a red light and a green light. Therefore, a RGB triplet LED array may be used as a high-bit-rate data transmitter and a lighting device at the same time.
FIG. 2A shows a schematic view of a general transmitter 200 using RGB triplet LEDs with a single input data stream 212. In the transmitter 200, the single input data stream 212 is received by an error correction encoder 214, and the output of a modulator 216 is applied to a triplet LED driver 218 directly. Three output powers PR, PG and PB of three primary colors (i.e. red, green and blue) are generated by the triplet LED driver 218. FIG. 2B shows an example of the output of the modulator with an on-off keying modulation scheme. The bit stream outputted by the modulator 216 is distributed to the triplet LEDs sequentially, wherein R, G and B represent three bit streams of three primary colors, respectively. FIG. 2C shows an example of the output powers of the triplet LED driver.
FIG. 3A shows a schematic view of a wavelength division multiplexing (WDM) transmitter 300 using RGB triplet LEDs with multiple input data. In the transmitter 300, each of multiple input data such as input data0, input data1 and input data2, has individual error correction encoder and modulator. The outputs of three modulators are coupled to three LED drivers directly. FIG. 3B shows an example of the output powers of the three LED drivers.
FIG. 4 shows a schematic view of a technique for visible light communication (VLC). As shown in the FIG. 4, a VLC apparatus may comprise a transmitting side device 410 and a reception side device 420. The transmitting side device 410 for transmitting a plurality of communication data (such as data 1, data 2, data 3, etc.) includes an illuminator 412 for generation of the illumination light, a communication amount adjuster 413 and a modulator 411. The communication amount adjuster 413 receives each of a plurality of communication data signals, and generates a dummy data for a corresponding one of the plurality of individual light sources so that data to be transmitted through each of light sources has an equal communication amount. The modulator 411 modulates each of the received communication data signals and the received dummy data into a driving signal for said each of the plurality of light sources.
FIG. 5 shows a schematic view illustrating a data transmitting apparatus using visible light communication. As shown in the FIG. 5, the data transmitting apparatus 510 may comprise a code generator 512 that converts transmitting data to a two-dimensional data code 520 having different colors and patterns according to times T0, T1, . . . , TN-1, TN, a modulation unit 514 that generate a modulation signal by modulation the two-dimensional data code, a plurality of LEDs 518 that are arranged in a two-dimensional form and emit light, and a light source driver 516 that controls the turn-on of the LEDs 518 according to the modulation signal.
From the aforementioned technologies, it may be seen that if the output of the modulator(s) or modulation unit(s) is applied to the LED driver(s) directly, the respective energy distribution of the three primary colors may be different from one another. Therefore, there may be an undesirable color tone in the situation. Also, the time varying luminance may be sensible by the human eyes when the transmitter serves as a lighting device at the same time. Light communication is still a potential solution to the global wireless spectrum shortage. Various solutions for visible light communication techniques have been suggested. There are challenges in these solutions for using an apparatus to perform image or video display and light communication simultaneously.